This invention relates to valve trains for internal combustion engines and more particularly to modified cam overlap profiles for control of internal exhaust gas recirculation or retention (EGR).
It is known in the automotive engine art to provide an engine having one or more cylinders with dual camshafts, one for actuating the engine intake valves and a second camshaft for actuating the engine exhaust valves. It is also known to provide a cam phaser or camshaft timing adjuster on one or both of the camshafts for adjusting within predetermined ranges the angular positions or phases of the camshafts relative to the engine crankshaft. A known arrangement is to mount a single cam phaser on the exhaust camshaft of the engine, although dual phasers may be utilized if desired.
In the referenced arrangement, the exhaust cam phaser adjusts timing of the exhaust camshaft during engine operation to vary the amount of EGR remaining in the engine combustion chambers during the compression and combustion processes. Dilution of the fresh air charges by exhaust gas during certain operating conditions of the engine is utilized to reduce the output of oxides of nitrogen (NOx) and to increase fuel economy. The variation in internal EGR is accomplished by increasing or decreasing the overlap of the exhaust valve closing motion and the intake valve opening motion by varying the relative timing of their cam profiles which actuate the valves through adjustment of the angular position of one or both of the camshafts.
In general, it is desired to dilute the engine cylinder charges with substantial amounts of retained exhaust gas. However, the amount of dilution must be controlled below an amount which will cause instability in the combustion process and adversely affect operation of the engine. Thus, the amount of overlap allowed during various engine operating conditions must be established by calibration to avoid worst case operating conditions which might cause instability.
The present invention results in part from a recognition that the problem of accurately controlling exhaust dilution by varying cam timing is made more difficult by the shape of the lift curves of the ordinary intake and exhaust valve cams. In general, both types of cams begin and end their opening and closing curves with relatively shallow ramps and acceleration or deceleration zones. In between, the curves accelerate and decelerate in more or less exponential fashion. Thus, with normal valve overlap, where increased cylinder charge dilution is not desired, the shallow deceleration portion of the exhaust valve overlaps the shallow acceleration portion on the intake valve for a relatively short distance, providing a relatively small maximum area of valve overlap and thus a small amount of dilution.
As the exhaust cam, for example, is retarded by the cam phaser, the overlapping portions of the lift curves quickly move into the exponential closing and opening portions of the exhaust and inlet cam profiles. The maximum overlap area accordingly rises quickly to a much greater amount than is initially available, so that a large change in the overlap area occurs with a relatively small motion of the cam phaser. This makes accurate control of the amount of EGR in the cylinders difficult and requires that the calibration settings be backed off for all engines to a position where none of the engines will operate in an unstable combustion condition even through there may be considerable variation in the amount of dilution occurring in the respective engines.
The present invention solves the problem by providing modified cam profiles which alter the rate of change of valve overlap area as the cam phase, or angular position, is adjusted over the range of cam phaser travel so that the total valve overlap area varies in a relatively linear manner as adjustment of the cam phase changes the duration of valve overlap. In this way, the more or less exponential relationship of EGR flow to cam phaser travel is replaced by a more linear relationship. Thus, the variation of exhaust gas retention in the combustion chamber becomes a relatively linear function of the duration of valve overlap in each engine cycle and control of the amount of EGR is improved
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.